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Join (SQL)

Join (SQL)
A programmer writes a JOIN statement to identify the records for joining. If the evaluated predicate is true, the combined record is then produced in the expected format, a record set or a temporary table. Relational databases are often normalized to eliminate duplication of information when objects may have one-to-many relationships. For example, a Department may be associated with many different Employees. Joining two tables effectively creates another table which combines information from both tables. This is at some expense in terms of the time it takes to compute the join. Note: In the Employee table above, the employee "John" has not been assigned to any department yet. This is the SQL to create the aforementioned tables. CROSS JOIN returns the Cartesian product of rows from tables in the join. Example of an explicit cross join: SELECT *FROM employee CROSS JOIN department; Example of an implicit cross join: SELECT *FROM employee, department; We can write equi-join as below,

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Chapter 10. Built In Functions Chapter 10. Built In Functions Fred Toussi The HSQL Development Group Copyright 2010-2012 Fred Toussi.

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